docs: def filter_ratio(df, label="", with_multiplicity=False):
      """
      Filter to include only the Ratio values with optional specified label, excluding other
      Intensity measurements, but retaining all other columns.
      """
      label += ".*__\d" if with_multiplicity else ""
  
      dft = df.filter(regex="^(?!Ratio).*$")
      dfr = df.filter(regex='^(.*Ratio.*%s)$' % label)
  
      return pd.concat([dft,dfr], axis=1)